1. Field of the Invention
The present invention concerns an antenna with three-dimensional coverage and electronic scanning, of the random spare volume array type.
2. Description of the Prior Art
There are several known types of antennas that make it possible to obtain a three-dimensional coverage (most usually a hemispherical or almost-hemispherical coverage) using a configuration of fixed elements combined with electronic scanning, namely an antenna wherein the shape of the radiation pattern (notably the direction of a major lobe) is modified by playing on the individual, adjustable phase shifts of the different elements forming the array.
The configuration most commonly used, in practice, to make an antenna such as this consists in distributing the different elementary antennas of the array over one or more reflecting surfaces such as, for example, the surface of a cylinder or a plurality of differently oriented panels.
These antennas, of the so-called surface array type, are not however satisfactory in all respects. For example:
the cylindrical surface array has the drawback of mediocre coverage for relatively great elevation angles, that is when the direction of the zenith is approached;
the multiple-panel antennas enable this drawback to be overcome by placing the different panels (generally four in number) on the faces of a truncated pyramid, thus enabling a relatively satisfactory hemispherical coverage to be obtained.
However, these multiple-panel antennas are relatively costly, for each panel, and hence each antenna of the array, works in only one quadrant (in the case of an antenna with four panels).
As a matter of fact, for a given direction of the major lobe, only one of the four panels is used, and the elementary antennas of the other three panels in no way contribute to the formation of the beam in this direction.
As a result, to have total azimuthal coverage available, the number of antennas and phase-shifter modules have to be quadrupled, thus correlatively placing a burden on the cost of the entire unit.
Besides, there is another known type of antenna with three-dimensional coverage and electronic scanning wherein, unlike in multiple-panel or cylindrical surface antennas, all the elementary antennas of the array take part in the formation of the beam and contribute to the gain of the antenna, irrespectively of the direction of the major lobe.
These antennas are so-called "volume" antennas wherein, unlike in surface antennas, the elementary antennas are no longer distributed on the surface of a given plane or given volume but within a volume, (generally a sphere).
The elementary antennas are distributed in this volume as unevenly as possible so as to minimize the mutual coupling among elementary antennas and thus attenuate the lobes of the array to the maximum extent. This condition is obtained by distributing the antennas in the volume according to a statistically isotropic random relationship of distribution and, furthermore, by providing for a mean spacing between elementary antennas that is, notably, greater than a half wavelength.
We thus speak of a "random spare array". In arrays such as this:
the sparing process enables economizing on the number of radiating elements for a given dimension of the array, i.e. for a given aperture of the array. It also enables a sharp reduction in the couplings among sources, which are frequent causes of deterioration in the performance characteristics of array antennas; and
the randomness enables the elimination of the lobes of the array inherent in regular structures with large pitches.
An antenna such as this has been described notably in the document DE-A-28 22 845.
More precisely, this document describes a so-called "crow's nest" antenna, namely an antenna formed by an array wherein the elementary antennas are open loops or turnstile antennas radiating on a horizontal polarization and placed at the top of the vertical coaxial lines of supply.
Although it appears to be an approach that is very worthwhile for an antenna with three-dimensional coverage and electronic scanning, this type of antenna, while having been proposed for more than 10 years, has been made only on an experimental basis until now, without any effective application to the different fields where an antenna of such a type might prove to be particularly desirable, namely fields such as those of air and naval defence, radar for weapons systems, secondary radars for aviation etc.
For, first of all, the length of the coaxial lines (of which the longest ones have a length equal to at least twice the radius of the enveloping sphere) makes the system mechanically weak. Hence, if it is desired to have the requisite precision of positioning of the different loops inside the sphere along with adequate overall rigidity, it becomes necessary to provide for additional mechanical mean such as nylon threads holding the semi-rigid supply cables in position and/or means to bury the entire array in a mass of foam (polyurethane foam for example).
In addition to the difficulties of mechanical implementation, in the latter case, the presence of foam plays the role of a thermal insulator which prevents the removal of calories if the antenna should be used in transmission. This point restricts this approach to low-power reception or transmission antennas and the problem of removal of calories is unresolved.
A second drawback, also related to the great length of the supply lines, is the inherent phase shift introduced by these lines. This phase shift may vary in great proportions (depending on whether the line is short or long) and it will be necessary to provide for compensation to prevent the appearance of phase errors independent of the direction of aim.
These drawbacks, both mechanical and electrical, related to the great length of the supply lines, are all the more inconvenient as the dimensions of the sphere are greater than the wavelength. Now, the fact that the narrowness of the beam (the angle of aperture of the major lobe) is directly related to the dimension (expressed in wavelengths) of the sphere leads to restricting the performance characteristics of the system as regards its beam narrowness.
Thirdly, a array such as this is highly "visible" in terms of radar signature, owing to the use of loops or turnstile antennas. Now, the use of such types of elementary antennas is inevitable because, by its nature, an array requires antennas with a pattern that is azimuthally almost omnidirectional in amplitude as well as in phase.
Fourthly, owing to its structure, this known type of antenna is restricted to working essentially in horizontal polarization.
Now, a great many applications absolutely call for a vertical polarization. This is the case, for example, with onboard radar antennas in ships (for, the vertical polarization gets rid of the effects of reflection on the sea) or again for antennas for secondary radars, notably IFF (Identification Friend or Foe) radars.
These different reasons explain the reason why, despite its obvious theoretical advantages and the need for an antenna with three-dimensional coverage and electronic scanning in many fields of application, this known type of antenna has not gone beyond the experimental stage until now.